Remove legacy in-app overage billing

Blocked on https://github.com/Flagsmith/flagsmith/issues/7970

Once we GA the Metered Billing integration, we can remove the legacy in-app overage billing.

This retires the [`charge_for_api_call_count_overages`](https://github.com/Flagsmith/flagsmith/blob/a7b0d217f714d4c59ae78023b3d7147d6c5da095/api/organisations/tasks.py#L159) chain and the 100K-addon quantity billing it drives.

## Acceptance criteria

- [ ] Flagsmith no longer computes or manages overage charges.
- The model `OrganisationAPIBilling` is sunset.
- The task `charge_for_api_call_count_overages` is retired.
- [ ] The `chargebee_metered_billing` feature flag is retired.

Start by reviewing blocked issue 7970 and the legacy chain in api/organisations/tasks.py, including charge_for_api_call_count_overages. Trace references to OrganisationAPIBilling, the 100K-addon quantity billing, and the chargebee_metered_billing feature flag. Done means the model, task, and flag are retired and Flagsmith no longer computes or manages overage charges.

Written by the indexing model from the issue text.